Armored magnetic field antenna in printed circuit

ABSTRACT

A shielded magnetic-field antenna has at least one turn of a metallic element and metallic tubular shielding positioned around the element. The turn ( 3   a ) is produced in a printed circuit on a card made of insulating material ( 4   a ). The shielding has two open rings ( 5   a,    6   a ) each produced in a printed circuit and disposed respectively on a bottom card ( 9   a ) and a top card ( 10   a ) made of insulating material. The cards are assembled by gripping the turn between the top and bottom cards. A plurality of metallic vias ( 7   a,    8   a ) connect side edges of the two rings. The vias extend through the cards, and are positioned around the turn.

The present invention relates to a shielded magnetic-field antenna ofthe type having at least one turn of a metallic element and a metallictubular shielding disposed around the said element.

It relates in particular to an antenna intended to be used inassociation with a chip card reader of the contactless type.

Two kinds of antenna of the above type are notably known. One, usuallyreferred to as a coaxial cable, is flexible, the shielding consisting ofa metallic braid disposed around a conducting wire sheathed withplastic; the other comprises shielding consisting of a rigid metallictube, made from copper for example, disposed around a conducting wirealso sheathed with plastic.

Producing these antennas has the drawback of requiring manual operationswhich are expensive and cannot be faithfully reproduced, such as anoperation of cutting the coaxial wire or copper tube to the correctlength, cutting the shielding in the middle of the loop opposite thelocation of a connection to an appliance, and then operations ofsoldering the antenna to a connector and to a frequency tuning system.

In addition, because of their design and the manual operations whichthey require, it has been found that such antennae exhibit disparitiesin magnetic characteristics compared with each other. The inventorsdeduced therefrom that they were not sufficiently reproduciblegeometrically.

Disparities in characteristics can also exist in an antenna when it issubjected to impacts or mechanical vibrations which cause it to changegeometrically.

The consequence of these disparities, in the first case, is to makeunsuitable a frequency tuning system with very fine adjustment designedto equip a series of antennae.

In the other case, the consequence of these disparities is to disturbthe antenna with respect to a frequency adjustment made during themanufacture or after this by means of an associated frequency tuningsystem.

This type of antenna used in a metallic environment requires theassociation of a ferrite element and a metallic screen disposedunderneath so as to give it immunity against surrounding magnetic fieldinterference.

Because of its fragility, the ceramic element must be protected fromvibrations and impacts which may occur against the antenna or screen. Itis therefore necessary to provide an assembly of the whole whichprotects the breakable element.

At the present time, the antenna and ceramic element are embedded inresin. This has the drawback of being inconvenient and expensive toimplement. In addition, the antenna obtained is not geometricallyreproducible.

The present invention aims to mitigate the drawbacks set out above.

The objective of the invention is therefore to design an antenna whichis reproducible, geometrically stable and of low cost.

Another objective of the invention is to design an antenna able to beused in a metallic environment which is insensitive to vibration andshock and whose design affords easy, reproducible and economicalmanufacture.

To this end, according to a preferred embodiment, the object of theinvention is a shielded magnetic-field antenna having at least one turnof a metallic element and metallic tubular shielding disposed around thesaid element.

It is characterised in that the turn is produced in a printed circuit ona card made of insulating material, such as a printed-circuit card, andin that the said shielding consists on the one hand of two open rings,produced in a printed circuit, and disposed respectively on a bottomcard and a top card made of insulating material, the said cards beingconnected by clamping the turn, and on the other hand a plurality ofmetallic vias distributed on each side of the turn, the said viasconnecting the edges of the two rings through the said cards. These viasare preferably uniformly distributed.

By virtue of such a design, the antenna is very rigid, and thereforevery stable geometrically compared with the antennae of the prior art.

According to another embodiment, for reasons of efficiency at lowfrequencies, the said shielding consists on the one hand of two openrings, produced in a printed circuit, and disposed respectively on abottom annular support and a top annular support made of insulatingmaterial, the said supports gripping the turn, and on the other hand twometallic films connecting the edges of the two rings on each side of theturn, the said films being deposited on the edges of the annularsupport.

By virtue of the characteristics of the above two embodiments, theantenna can be manufactured on an industrial scale by theprinted-circuit technique. This technique, perfectly mastered, ensuresgood reproducibility of its geometric characteristics. This can beeffected at lesser cost since the manufacture uses production toolswhich are generally standardised and automated.

In addition, this design makes it possible to change the format of theantenna very easily since the majority of the operations are automated.

Another object of the invention is a device with a magnetic-fieldantenna including a printed-circuit antenna according to the invention,a ferrite layer, a metallic screen, two flexible leaves disposedrespectively between the antenna and the ferrite layer on the one handand between the ferrite layer and the metallic screen on the other hand.These leaves can have an adhesive on their faces in order to facilitateassembly.

The design of this device by assembling diverse layers has the advantageof adapting the antenna very easily to a metallic environment subjectedto mechanical impacts or vibrations.

According to other preferred characteristics, the device can includeclamping means for assembling and clamping together the ferrite layer,the flexible leaves and the metallic screen.

By virtue of these provisions, the elements can easily be connectedtogether and if necessary dismantled, and the distance separating themchecked.

According to other preferred characteristics, the device has means forkeeping the ferrite at a distance from the antenna and keeping thescreen at a distance from the ferrite.

By virtue of these provisions, the reproducibility of the geometricparameters of the device is also ensured.

Other characteristics and advantages of the invention will emerge fromthe following description, given solely by way of example, in no waylimitative, referring to the accompanying drawings, in which:

FIG. 1 depicts an antenna according to a first embodiment;

FIG. 2 depicts an antenna according to another embodiment;

FIG. 3 is a transverse section of the antenna of FIG. 1 along 3—3,illustrating its cross section;

FIG. 4 is a transverse section of the antenna according to FIG. 3, thisbeing in the course of assembly;

FIG. 5 is a transverse section of the antenna of FIG. 2 along 5—5,illustrating its cross section;

FIG. 6 is a transverse section of the antenna according to FIG. 5, thisbeing in the course of assembly;

FIG. 7 depicts printed-circuit cards used for producing the antenna withthe location of a complementary circuit;

FIG. 8 depicts the constituents of the antenna of FIG. 2;

FIG. 9 depicts a detailed plan view of the antenna according to FIG. 2;

FIG. 9a is an enlarged detail, of a portion of the antenna of FIG. 9;

FIG. 10 depicts a disassembled view illustrating the assembly of thedifferent constituents of the antenna device according to the invention.

In FIGS. 1 and 2, first and second embodiments of an antenna accordingto the invention can be seen respectively. A description will be givenfirst of all of the first embodiment with the help of FIGS. 1, 3 and 4.

According to a first embodiment, the antenna has a general loop shapeand has a rectangular cross section A—A. This cross section helps toreinforce its geometric stability. Preferably the loop is circular as inthe example.

In accordance with the invention, the shielded magnetic-field antennahas at least one turn 3 b of a metallic element and metallic tubularshielding 5 b, 6 b, 7 b, 8 b disposed around the said element.

In the example, the antenna has a single thin flat copper element 3 b,disposed at the centre of a tubular structure with a rectangular crosssection 5 b, 6 b, 7 b, 8 b. The element 3 b is sandwiched between tworings 9 b, 12 b made of insulating material and with a rectangular crosssection 10 b, 9 b, preferably with the same thickness. In this case theinsulating material is epoxy glass.

On each side of the element 3 b, between the two rings 9 b, 12 b,another insulant such as air can be found, or an adhesive connecting thetwo rings.

According to one variant, one of the two rings 12 b consists of tworings 4 b and 10 b connected together (FIG. 4), for example by gluing.

As for the tubular shielding, this consists of a thin metallic film, forexample a 35 μm copper film.

In FIG. 1, it can be seen that the shielding ring is open. It has abreak 17 b forming an air gap necessary to the correct functioning ofthe antenna in accordance with a known teaching, the said air gap beingdisposed opposite the connection points 14 b of the antenna so that theshielding arms have strictly the same length.

In FIGS. 3 and 4, it can be seen that the antenna has at least twoannular insulating supports: a first top support 4 b having the turn 3 bon one of its parallel faces, a portion 6 b of the shielding being onthe other face, and a second bottom support 9 b having solely a portion5 b of the shielding.

Once the two annular supports have been assembled, for example bygluing, their edge 11 receives a metallisation 7 b, 8 b which connectsthe portions 6 b and 8 b of each side of the element. The metallisationcan be effected by any thin film deposition method, for example byspraying or mechanical deposition.

These metallic elements and the supports are advantageouslyprinted-circuit elements. Consequently, it will be understood thatproducing the shielding uses the technique of printed-circuitmanufacture.

According to a variant, only the turn 3 b is produced initially, themetallic film 5 b to 8 b is being produced together thereafter, forexample by spraying.

Another preferred embodiment will now be described with the help ofFIGS. 2, 5, 6.

According to this embodiment, the antenna 1 has a general continuousshape in block form, which affords very good geometric stability.

It consists of several cards made of insulating material, three in FIG.2 or preferably two in FIGS. 5 and 6.

In the same way as before, it has a turn of a metallic element 3 a and ametallic tubular shielding 5 a, 6 a, 7 a, 8 a disposed around the saidelement.

The turn is identical to the turn in the previous example. On the otherhand, there are differences in the support and the lateral walls of theshielding. The supports are continuous cards whilst the lateral wallsconsist of a plurality of metallic vias 7 a and 8 a connecting the tworings 5 a and 6 a.

These vias or cross members are distributed on each side of the turn 3 aalong the latter (FIG. 9). Preferably, these vias are spaced apart by assmall a distance as possible so as to be effective at low frequencies.In the example, this distance is equal to 2.5 mm, the diameter of thevias being 0.5 mm; this distance gives the antenna a goodefficiency/strength ratio in this example, which relates to the readingof contactless chip cards.

The vias pass through the cards 9 a, 12 a and electrically connect thelateral edges of the metallic rings of the shielding.

As before, the card 12 a carrying the turn can be produced from twodistinct cards 4 a, 10 a (FIG. 6) connected by gluing subsequently orfrom a single card 12 a, whilst the other card 9 a carries solely abottom portion of the screening 5 a.

As before, it can be seen in FIG. 6, according to a preferredembodiment, that the turn 3 a and the ring 6 a are produced first of allon the same printed circuit card 12 a, whilst the other ring is alsoproduced in a printed circuit on another card.

These cards 9 a and 12 a are then assembled, for example by gluing.

In a last operation, orifices are pierced all along the lateral edges ofthe metallic rings in which vias 7 a, 8 a are produced, like the onesnormally used in the production of printed circuits. These vias can bereplaced by any kind of elongate metallic element providing the sameelectrical connection function, for example hollow or solid rivets.

Advantageously, the plurality of vias constitute a perforated wall ofthe screening in the same way as a braid of a coaxial cable; ittherefore fulfills a similar function.

Through the rigidity and geometric stability of the printed circuitcards, a particularly stable and reproducible antenna is obtained.

Advantageously, the location 14 a of the connection and the location ofthe tuning circuit 13 a are effected on the same support as that of theantenna.

FIG. 7 illustrates the metallisations produced on three printed circuitcards 10 a, 4 a and 9 a. At least three metalisations are necessary: onemetallisation for the top ring of the shielding 6 a and forcomplementary circuit elements 13 a such as the location of a tuningcircuit 14 a and of a connector 15 a, another metallisation 3 a for theturn, and a last one for the ring 6 a and the above complementarycircuit elements 13 a.

Thus it is possible to create the antenna and its complementaryelectrical circuit elements in three printing operations. It sufficesthereafter to assemble the three cards produced separately, to place thevias on the card in an automated fashion, and then to connect it and thecomponents of the tuning circuit including at least one variablecapacitor with very fine adjustment.

The card can have orifices 16 a enabling metallic inserts such asspacers to be introduced subsequently.

By virtue of the use of a widespread manufacturing method, it ispossible to produce such an antenna easily and rapidly on an industrialscale. In addition, it is also easy to change format according to theenvisaged applications.

FIG. 8 illustrates the three printed-circuit cards 4 a, 9 a, 10 aobtained here without the location of the tuning circuit. On the otherhand, they have respectively a ring 5 a with a location 14 a forreceiving a connector, a turn 3 a and a ring 6 a with another location14 a.

In FIG. 9 and FIG. 9a, it can be seen that the vias 7 a, 8 a aredistributed along edges of the ring and that the latter has an air gap17 a, disposed diametrically opposite the connecting points 14 a.

In FIG. 10, a magnetic-field antenna device 30 includes aprinted-circuit antenna 2. In the example, it is a case of the antennaaccording to the invention. In this figure the antenna is equipped withits tuning circuit 23 and a connector 24.

The device 30 also has shielding by means of a material able to channelelectromagnetic waves, such as for example a ferrite plate 18 consistingof a set of flat ferrite bars 19 disposed against each other and ascreen against electromagnetic waves such as a steel plate 20.

The assembly consisting of ferrite and screen is disposed successivelybelow the antenna 2 in FIG. 10; they must be interposed between theantenna and the metallic surroundings. Such an arrangement isolates theantenna from a metallic environment which could disturb it.

According to the invention, the ferrite assembly 18 is isolated from theantenna 2 of the invention by means of a leaf 21 of non-magneticcompressible material able to damp the mechanical vibrations or impact.The same material, in the form of a leaf or layer 22, is disposedbetween the screen 20 and the ferrite assembly. The material can becompact such as rubber or Neoprene-based foam.

In the example, Neoprene foam leaves have been used, advantageouslyhaving adhesive faces so as to facilitate mounting.

Thus it is possible to assemble different breakable elements such asferrite or ceramic with a printed circuit element such as the antenna 2of the invention.

The antenna device 30 can also have remote adjustment means 25 for moreprecisely controlling the distances between the different layers formedby the printed-circuit antenna, the ferrite and the screen. They canalso have clamping means 26, 27 for controlling the damping and theclamping together of the layers.

In the example, the antenna device 30 has spacer pads 25 made from adenser material than the foam, for example ten times denser andtherefore ten times less compressible under the same pressure, thisbeing disposed laterally between the steel screen 20 and the printedcircuit 2 so as to keep a substantially constant distance between them.The pads 25 can have orifices to enable the clamping means mentionedbelow to pass.

The antenna device preferably has assembly and clamping means consistingin the example of four screws 26 and corresponding nuts 27 disposed atthe four corners of the antenna 2, the nuts being integrally fixed tothe screen.

As a variant, sockets 27 can be used which have the function of nut andspacer. Thus, in tightening the screws completely there is always thesame separation and the same compression of the leaves. The antenna 2can also have annular metallic inserts 28 which serve both as a washerfrom the screws 26 and which have a function of spacer for theprinted-circuit card.

Where applicable, the antenna can have a sole plate 29 made ofnon-magnetic flexible material such as rubber on which the completeantenna can bear whilst being isolated from the vibrations of thereceiving support.

By virtue of these characteristics, the antenna is geometrically stable,since it is not disturbed by impacts or vibrations. In addition, thebreakable elements being protected, it offers excellent mechanicalstrength.

Moreover, it is possible to manufacture it simply by assembling, andthis in a reproducible fashion.

In this application, the adjustable capacitors of the tuning circuit canalso be chosen with a very fine adjustment.

What is claimed is:
 1. A shielded magnetic-field antenna having a turn(3 b) of a metallic element and metallic tubular shielding (5 b, 6 b, 7b, 8 b) disposed around said element, characterized in that the turn (3b) is produced in a printed circuit on a first annular support made ofinsulating material (4 b), and in that said shielding comprises two openmetallic rings (5 b, 6 b) produced in a printed circuit respectively ona bottom annular support (9 b) and a top annular support (10 b) made ofinsulating material, said top and bottom supports gripping the turn, andtwo metallic films (7 b, 8 b) connecting the two rings on each side ofthe turn, said metallic films being deposited on side edges (11) of theannular supports.
 2. An antenna according to claim 1, characterized inthat the turn (3 b) and one of said rings (6 b) are disposed on a commonannular support (12 b).
 3. A shielded magnetic-field antenna (2) havingat least one turn (3 a) of a metallic element and metallic tubularshielding (5 a, 6 a, 7 a, 8 a) disposed around said element,characterized in that the turn (3 a) is produced in a printed circuit ona card made of insulating material (4 a) and in that said shieldingcomprises two open rings (6 a) produced in a printed circuit anddisposed respectively on a bottom card (9 a) and a top card (10 a) madeof insulating material, said top and bottom cards (9 a, 10 a) beingassembled by clamping the turn (3 a), and a plurality of metallic vias(7 a, 8 a) connecting the edges of the two rings (5 a, 6 a) through saidcards, said vias being distributed on each side of the turn.
 4. Anantenna according to claim 3, characterized in that the turn (3 a) andone of said rings (5 a, 6 a) are disposed of a command card (12 a). 5.The antenna according to claim 4 wherein the vias (7 a, 8 a) are spacedapart by a distance of approximately 1.5 mm.
 6. The antenna according toclaim 4 wherein the printed-circuit card also has an location (13 a, 14a) for a tuning system (33) and for a connection (24).
 7. An antennaaccording to claim 3, characterised in that the vias (7 a, 8 a) arespaced apart by a distance of approximately 1.5 mm.
 8. The antennaaccording to claim 7 wherein the printed-circuit card also has anlocation (13 a, 14 a) for a tuning system (33) and for a connection(24).
 9. An antenna according to claim 3, characterised in that theprinted-circuit card also has a location (13 a, 14 a) for a tuningsystem (33) and for a connection (24).
 10. A magnetic-field antennadevice (30) having a printed-circuit antenna (2) according to one of thepreceding claims 1, 2, 3, or 4, further comprising a ferrite layer (18),a metallic screen (2), two flexible leaves (21, 22) disposedrespectively between the antenna (2) and the ferrite layer, and betweenthe ferrite layer and the metallic screen.
 11. The device according toclaim 10, further comprising clamping means (26, 27) for assembling andclamping together the antenna (2), the ferrite layer (18), the flexibleleaves (21, 22) and the screen (20).
 12. The device according to claim11 further comprising spacers (25, 27, for keeping the ferrite (18) at adistance from the antenna (2) and keeping the screen (20) at a distancefrom the ferrite (18).